Managing an Entity Instance’s Life Cycle

You manage entity instances by invoking operations on the entity by
means of an EntityManager instance. Entity instances
are in one of four states: new, managed, detached, or removed.

New entity instances have no persistent identity and are not yet associated
with a persistence context.

Managed entity instances have a persistent identity and are associated
with a persistence context.

Detached entity instances have a persistent identify and are not currently
associated with a persistence context.

Removed entity instances have a persistent identity, are associated
with a persistent context, and are scheduled for removal from the data store.

Persisting Entity Instances

New entity instances become managed and persistent either by invoking
the persist method, or by a cascading persist operation
invoked from related entities that have the cascade=PERSIST or cascade=ALL elements set in the relationship annotation. This means
the entity’s data is stored to the database when the transaction associated
with the persist operation is completed. If the entity
is already managed, the persist operation is ignored, although
the persist operation will cascade to related entities
that have the cascade element set to PERSIST or ALL in the relationship annotation. If persist is
called on a removed entity instance, it becomes managed. If the entity is
detached, persist will throw an IllegalArgumentException, or the transaction commit will fail.

Removing Entity Instances

Managed entity instances are removed by invoking the remove method,
or by a cascading remove operation invoked from related
entities that have the cascade=REMOVE or cascade=ALL elements
set in the relationship annotation. If the remove method
is invoked on a new entity, the remove operation is ignored,
although remove will cascade to related entities that have
the cascade element set to REMOVE or ALL in the relationship annotation. If remove is
invoked on a detached entity it will throw an IllegalArgumentException, or the transaction commit will fail. If remove is
invoked on an already removed entity, it will be ignored. The entity’s
data will be removed from the data store when the transaction is completed,
or as a result of the flush operation.

In this example, all LineItem entities associated
with the order are also removed, as Order.getLineItems has cascade=ALL set in the relationship annotation.

Synchronizing Entity Data to the Database

The state of persistent entities is synchronized to the database when
the transaction with which the entity is associated commits. If a managed
entity is in a bidirectional relationship with another managed entity, the
data will be persisted based on the owning side of the relationship.

To force synchronization of the managed entity to the data store, invoke
the flush method of the entity. If the entity is related
to another entity, and the relationship annotation has the cascade element
set to PERSIST or ALL, the related entity’s
data will be synchronized with the data store when flush is
called.

If the entity is removed, calling flush will remove
the entity data from the data store.